This invention relates to an I/Q modulator or I/Q demodulator circuit.
The prior art is described below with reference to FIG. 1.
FIG. 1 is a circuit diagram of a generally used circuit for an I/Q-modulator or I/Q-demodulator. The circuit comprises an RF-transformer 1 and a diode bridge formed by four diodes 4, 5, 6, 7, and two low pass filters formed by a pair of capacitors 8, 9 and 11, 12, respectively, and by a resistor 10 and 13, respectively.
The input signals 2 and 3 are lead through the low pass filters to the diode bridge formed by the four diodes 4-7. The local oscillator (LO) signal is lead to the diode bridge through the RF transformer 1. The mixed signals are lead to capacitors 14, 15 and then to a capacitor 17 and a resistor 18. A resistor 16, a capacitor 19 and a coil 20 are connected to ground. A combined MOD OUT -signal is taken as an output signal through a capacitor 21.
The modulating signal is lead to the modulator as a separate branch as shown in FIG. 1. In the modulator according to FIG. 1, however, the diodes have to be close to the RF-transformer and clog to each other. The layout becomes difficult, since the diodes have to receive signals from two branches.
Also chokes should be used in the place of resistors 10, 13 so that the modulating signal branch would not load the output at the output frequency.